It is a bad idea only cause it will temporarily distort the perfectlocality of the regions hosted by each RegionServer. This getscorrected only at the end of the next major compaction of all regions,eventually, but both the events would cause some small level ofperformance dips and increase in network use + I/O until done.

There's no way to escape the fact that if you write more HBase data,the 3 nodes of RS are bound to fill up faster than the others, butwhat we could do as an enhancement for aiding rebalancing theremaining replica nodes without affecting the RS is to provide anexclude-nodes feature to the balancer. By asking the Balancer toexclude the RS's nodes, you can rebalance the rest of the clusterwhile not causing a performance problem on the RS during the time.

Most clusters run the RS+DN pair across all nodes, so this scenario ofan imbalance won't really occur there.